1. Field of the Invention
The field of the invention is resolver excitation circuits and more particularly methods of precisely synchronizing the phase of a cosine and sine wave to digital reference signal.
2. Description of the Prior Art
Resolvers are used to provide an electrical measure of angular position or related quantities such as velocity or acceleration. A resolver is similar in construction to a motor, containing a stationary set of two perpendicularly oriented stator coils and a rotatable rotor coil. When the stator coils are excited by properly phased sinusoidal signals, the rotor coil generates a sine wave signal whose phase, with respect to the stator signals, is directly related to the rotor's angular position within the stator. If the phase of the stator signals is known, the angular position of the rotor may be derived from the phase of the rotor's signal.
The accuracy of the waveforms used to excite the stator of the resolver affects the accuracy of the resolver's operation. Typically the two coils of the resolver's stator are excited by sinusoidal waveforms with a 90.degree. phase difference: i.e. a sine and cosine signal. If the sine and cosine signals are of constant frequency, exactly 90.degree. in phase difference, of equal amplitude, and of low distortion, then the phase of the sinusoidal rotor signal will accurately reflect the rotor's angular position. Phase or amplitude variation in the exciting sine and cosine signals or distortion in their waveform shapes will decrease the resolver's accuracy.
The generation of pure sinusoidal signals with precise frequency and phase relationships is extremely difficult with analog circuitry. Accordingly, digital synthesis techniques are typically used for the generation of resolver excitation signals. Such digital synthesis techniques store the values of the waveform to be synthesized, over a full waveform cycle, in a digital memory "look-up table". The look up table is read sequentially at a high rate into a digital-to-analog converter ("DAC") which produces a "staircase" approximation of the desired waveform. This staircase waveform is then typically filtered, by a low-pass filter network, to remove the staircase "distortion" and to produce a pure sine or cosine wave.
Unfortunately, the low-pass filter network can introduce significant phase and amplitude error into the digitally synthesized sine and cosine signals. These errors result both from phase "lag" or "lead" intrinsic to the filtering process and from drift of the values of the analog components of the filter network and from variation within each component's tolerance.